Method of processing disseminated chromite ores



" Dec. 4, -1956 A. M. THOMSEN 2,772,957

METHOD OF PROCESSING DISSEMINATED CHROHITE ORES Filed Nov. 9. 1953 Obacsaf er all Jim, far Jflmrptiaru I/WIENTWZ.

United States Patent O METHOD OF PROCESSING DISSEMINATED CHROMITE ORES Alfred M. Thomsen, San Francisco, Calif., assignor to American Chrome Company, a corporation of Nevada Application November 9, 1953, Serial No. 391,023

2 Claims. (Cl. 75-1) While chromite is often found in nature in the form of massive lenses and segregation deposits consisting esdominant mineral is generally Olivine or related minerals.

Normally such ores are worked directly by some type of ore dressing in which case the tailings from such an operation are advantageously worked by my process. In other cases, particularly in the field of low-chrome ores,

or where nickel is also involved, my process commences directly with the crude ore.

Inasmuch as my process resides in the decomposition and removal of the complex silicates of the groundmass, thus liberating the disseminated chrome retained within same, it constitutes a continuation-in-part of an application filed on December 17, 1951, bearing Serial No. 262,181, now abandoned, and called Resolving of Complex Silicates. While the technique disclosed herein is identical in many respects with that of my previous application, yet the inclusion of chromite and a phase of the silica requires the filing of the instant application.

The aim and object of my process is to obtain the magnesia resident in said silicates as a principal product, the silica in a partially dehydrated form for use as an adsorbent, any nickel and chromium present being regarded more as by-products than as a principal product. In addition, it is inevitable that a certain amount of iron and aluminum will also be present as nature rarely presents pure minerals in an abundant manner, but such items will be regarded solely as impurities'that must be removed or that may remain as a part of the finished products previously enumerated. It is obvious that considerable iron and alumina can be present in a refractory grade of magnesia while a high grade magnesia will demand their complete elimination.

As a preferred illustration of my process I have, therefore, chosen a type of ore where the groundmass consists essentially of magnesium silicate containing a little nickel, iron and alumina with some chromium in the form of disseminated grains of chromite. The process is best understood if followed upon the drawing which will now be explained so fully that any operator can easily obtain the results indicated on said drawing.

I commence with showing the ground ore mixed with sulphuric acid in a mixer. The alternative use of ammonium bi-sulphate will be referred to at a later stage in my explanation. The acid is conveniently used at a dilution to 50% strength, and in an amount sufiicient to combine with substantially all the magnesia, iron and alumina being considered as consuming an equivalent amount. This is best done empirically by scanning the result of a later step, namely the behavior of the dissolver. When the magnesium sulphate solution is substantially neutral and when the suspended solids are substantially silica and unaffected chromite the acid treatment has been satisfactory.

, the scope of this disclosure.

If the ore employed be tailings from a previous milling operation for chrome, then such tailings may be used directly in their wet form and the water retained therein computed as a portion of the diluting water of the acid. If the ore be untreated mineral, then a preliminary grinding operation is mandatory. In general a 20 mesh' product Works quite satisfactory.

The position of the drier, interposed between mixer and dissolver, is most important. Chromite is not attacked to any extent by dilute acid so remains unaltered. When evaporation in the drier has removed the diluting water the acid has already combined with magnesia and other bases so once more the chromite remains inert. If an excess of acid were used by error the solution in the dissolver would turn green from dissolved chromium sulphate, thus giving warning to reduce such excess.

Thedrier serves a mechanical as well as the above described chemical purpose. The silica separated in the acid treatment is gelatinous in character and would prove most troublesome in separation of chromite as well as in washing to remove magnesium sulphate. By heating the acid mixture to approximately 500 F. the silica becomes partially dehydrated and thus filters well. In addition, drying in the presence of much magnesium sulphate conveys a porous. structure to the residual silica when leaching has removed said magnesium sulphate. Such washed silica will require only drying to fit it for the market and it. will do in oil processing virtually the work normally being done by acid treated clay, technically referred to as activated. Presumably it might serve to remove chromite and magnesium sulphate without such a drying step but to convey adsorption properties to the silica said drying step is mandatory.

After drying, in the dissolver which is but a device with effective agitation, the product is commingled with water and resolved into a sludge of dissolved sulphates and insoluble components. On thedrawing I have indicated a hydraulic separation between a chrome sludge and a silica sludge, hydraulic fluid to the classifier being furnished by recycled magnesium sulphate solution derived from the separator. Theme of classifiers in milling makes any description unnecessary. The separator may be a filter, or a centrifuge, or a series of decantation tanks or any other device whereby separation is made between the liquid and solid constituents of a slurry. Similarly other means, such as direct tabling, might be substituted for the classifier, or used in a supplementary manner, without in any way departing from the theme of this disclosure. 7

Further processing of the chromite sludge is beyond The separated silica requires but a drying step to make it acceptable for the market. That leaves as the sole remaining item the solution of magnesium sulphate containing such impurities as have been converted'to water-soluble sulphates in the acid treatment. The next step is purification from said impurities.

I have indicated the. addition of ammonium sulphide in the precipitator which is but a tank fitted with an agitator. When a black precipitate is no longer obtained on adding more sulphide the precipitation of iron and nickel will be about complete and but little alumina will 1 remain. For technical purposes this is adequate purification. Separation is next made between the purified magnesium sulphate solution and the insoluble sulphides. If enough nickel, cobalt, or other material is present in such sulphides they are worked for their constituents in orthodox manner which is outside the scope of this disclosure.

The purified magnesium sulphate solution is next commingled in another precipitator with sufficient ammonium carbonate to precipitate the resistant magnesium as car- Patented Dec. 4, 1956 bonate. no longer produce a precipitate optimum eifect will have been obtained. Separation is next made between insoluble magnesium carbonate and the residual solution of ammonium. sulphate, the latter being evaporated and crystallized in order to obtain said ammonium sulphate in marketable form.

If market conditions for sulphuric acid, ammonia, and ammonium sulphate are such at the point of origin of the ore that economic considerations permit of selling said ammonium sulphate at a profit then the process, as so far disclosed, is an acceptable solution. If the reverse be true then the modification represented by the substitution of ammonium bisulphate for sulphuric acid can advantageously be employed. I have represented this on the drawing as an alternative to the use of sulphuric acid.

Inasmuch as ammonium sulphate starts to decompose when heated to 150 C. and as decomposition is complete at 350 C. it becomes a simple matter to recycle sulphuric acid and thus avoid its expense. The ammonia evolved in heating is, of course, recycled to the process. I have indicated this step in the decomposer which yields simultaneously a fused ammonium bisulphate and gaseous ammonia. I have shown the former re-cycled to the mixer and the ammonia carbonated with the waste gas from the kiln in which the magnesium carbonate was calcined to MgO. While there is some market for magnesium carbonate the great outlet is that form of magnesia referred to as the refractory grade. In general, therefore, such a kiln will be in use and hence its waste gas will be available. Any other type of waste flue gas can obviously be substituted if it be dispensed with.

Following this directive the operator will readily make such modifications as irregularities of the particular ore may involve. Attention will be called to a few of these. It may be assumed that a certain amount of fine chromite will be retained in the silica Where it will be harmless. Should it be desirable to remove such chromite, then a re-treatment of the impure silica with strong sulphuric acid and drying the mixture once more prior to leachingwill result in placing both the iron and the chromium in solution as sulphates While a purified silica remains. Manifestly, conventional means can then be used for further utilization of the solution so it may be considered as outside of this disclosure. i

In the preferred version I have shown all impurities present in the magnesium sulphate solution that have insoluble sulphides as removed, in toto, for subsequent use or just for the purpose of purifying said solution. In the event that much iron be present, such as in laterites, it may be desirable to remove most of the iron first, by oxidation and precipitation, and then proceed with the sulphide step to obtain resident nickel. Inasmuch as this technique, in an ammonia cycle, is already fully described in the previous application of which this is a continuation, no further elucidation is needed.

Inasmuch as separation of chromite from the complex silicates of the groundmass is a primary objective of my process sundry combinations of the items that constitute this disclosure, but in a different order, will suggest themselves to any operator. Thus a very light treatment with acid will result in the removal of little but the colloidal slime in the product and this may be satisfactory to pro- Obviously, when an addition of carbonate will even be more desirable.

duce an adequate recovery of chromite. While much magnesium silicate will thus remain undecomposed it will have become superficially covered with activated silica.

While such a mixture of activated silica and residual magnesium silicate would be less active than a pure product such as previously described it would be more grantilar in physical appearance and might, in certain instances, All. such minor modifications I regard as being within the scope of my disclosure.

Having thus fully described my process, I claim:

1. In a process for resolving a complex ore wherein disseminated chromite is scattered through a matrix composed of a complex metal silicate, the steps consisting in mixing the ore with a suificient amount of an aqueous solution of ammonium bisulfate to decompose and com bine with substantially all of the metal present except chromium, but insufficient to react with the chromite present, whereby there is formed a mixture of metal sulfates, chromite and released silica, heating the mixture to about 500 F. for a period sutficient to dehydrate partially the liberated silica, adding water to the heated mixture to dissolve soluble metal sulfates present and provide a sulfate solution and the silica as one phase and chromite as a solid phase, and separating the two phases.

2. In a process for resolving a complex ore wherein disseminated chromite is scattered through a matrix composed of a complex metal silicate, the steps consisting in mixing the ore with a sufficient amount of an aqueous solution of ammonium bisulfat e to decompose and combine with substantially all of the metal present except chromium, but insufficient to react with the chromite present, whereby there is formed a mixture of metal sulfates, chromite and released silica, heating the mixture to about 500 F. for a period sufficient to dehydrate partially the liberated silica, adding water to the heated mixture to dissolve soluble metal sulfates present and provide a sulfate solution. and the silica as one phase and chromite as a solid phase, separating the two' phases, adding ammonium carbonate to the separated liquid phase to form ammonium sulfate, recovering the so-formed ammonium sulfate and returning it to the first of the foregoing steps.

References Cited in the file of this patent UNITED STATES PATENTS 1,403,237 Eustis Jan. 10, 1922 1,504,549 Govers Aug. 12, 1924 1,884,709 Jenkins et a1. Oct. 25, 1932 1,898,774 Guthrie et al. Feb. 21, 1933 2,070,497 Sweet et al Feb. 9, 1937 2,402,370 Chalmers June 18, 1946 2,601,306 Lloyd et a1. June 24, 1952 FOREIGN PATENTS 459,058 Great Britain Jan. 1, 1937 524,142 Great Britain July 31, 1940 619,160 Great Britain Mar. 4, 1949 118,265 Australia Dec. 19, 1946 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic Theoretical Chemistry, vol. 2,. page 702,- 1922. 

1. IN A PROCESS FOR RESOLVING A COMPLEX ORE WHEREIN DISSEMINATED CHROMITE IS SCATTERED THROUGH A MATRIX COMPOSED OF A COMPLEX METAL SILICATE, THE STEPS CONSISTING IN MIXING THE ORE WITH A SUFFICIENT AMOUNT OF AN AQUEOUS SOLUTION OF AMMONIUM BISULFATE TO DECOMPOSE AND COMBINED WITH SUBSTANTIALLY ALL OF THE METAL PRESENT EXCEPT CHROMIUM, BUT INSUFFICIENT TO REACT WITH THE CHROMITE PRESENT, WHEREBY THERE IS FORMED A MIXTURE OF METAL SULFATES, CHROMITE AND RELEASED SILICA, HEATING THE MIXTURE TO ABOUT 500* F. FOR A PERIOD SUFFICIENT TO DEHYDRATE PARTIALLY THE LIBERATED SILICIA, ADDING WATER TO THE HEATED MIXTURE TO DISSOLVED SOLUBLE METAL SULFATES PRESENT AND PROVIDED A SULFATE SOLUTION AND THE SILICIA AS ONE PHASE AND CHROMITE AS A SOLID PHASE, AND SEPARATING THE TWO PHASES. 